Emit code16 in assembly in 16-bit mode

[niravd]

Summary

When generating assembly using -m16 we must explicitly mark it as
16-bit. Emit .code16 at beginning of file. Fixes wrong results when
using -fno-integrated-as.

[dwmw2]

LGTM. Thanks.

[t.p.northover]

I didn't think we supported code16 compilation (only assembly). This appears to be the very first CodeGen test mentioning the triple and there's basically no reference to it in the actual X86 code.

Tim.

lib/Target/X86/X86AsmPrinter.cpp

if (TT.getArch() == Triple::x86) {
       // cause the process to terminate immediately.  LLVM does not know how to
       // register any SEH handlers, so its object files should be safe.
       OutStreamer->EmitSymbolAttribute(S, MCSA_Global);
       OutStreamer->EmitAssignment(
           S, MCConstantExpr::create(int64_t(1), MMI->getContext()));
     }
   }
   OutStreamer->EmitSyntaxDirective();
+
+  // If this is not inline asm and we're in 16-bit
+  // mode prefix assembly with .code16.
+  bool is16 = TT.getEnvironment() == Triple::CODE16;
+  if (M.getModuleInlineAsm().empty() && is16)
+    OutStreamer->EmitAssemblerFlag(MCAF_Code16);
 }
 
 static void
 emitNonLazySymbolPointer(MCStreamer &OutStreamer, MCSymbol *StubLabel,
                          MachineModuleInfoImpl::StubValueTy &MCSym) {
   // L_foo$stub:
   OutStreamer.EmitLabel(StubLabel);
   //   .indirect_symbol _foo

test/CodeGen/X86/x86-16.ll

+; RUN: llc < %s | FileCheck %s
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i386-unknown-linux-code16"
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  ret i32 0
+}
+
+; CHECK: .code16
+; CHECK-LABEL: main
+
+
+attributes #0 = { nounwind }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 3.9.0 (trunk 265439) (llvm/trunk 265567)"}
 No newline at end of file